This invention relates to a flowmeter incorporating a measuring tube, an ultrasound transducer, an ultrasound waveguide and a seal, said ultrasound transducer connecting outside the measuring tube to the ultrasound waveguide in such fashion that ultrasound waves generated by the ultrasound transducer can be transferred to the ultrasound waveguide and, conversely, ultrasound waves received by the ultrasound waveguide can be transferred to the ultrasound transducer, with the ultrasound waveguide protruding at least partly into the measuring tube.
A flowmeter of this type may be an ultrasound flowmeter or a vortex flowmeter. The ultrasound transducers employed are typically piezoelectric crystals capable of generating and/or detecting ultrasound waves.
There are applications for which it is possible to equip the flowmeter with only an ultrasound transducer without an ultrasound waveguide, the ultrasound transducer serving to generate as well as detect ultrasound waves. In any such design, the ultrasound transducer must be directly positioned at the point where the ultrasound waves are introduced or detected. That, however, tends to create a problem insofar as the piezoelectric crystals which, as pointed out above, are typically used as the ultrasound transducers in flowmeters, cannot be used above a certain temperature, the so-called Curie temperature. This is because above the Curie temperature, the crystal no longer possesses a ferroelectric or ferromagnetic phase, the prerequisite for the piezoelectric properties of the crystal.
Therefore, in cases where for instance the moving fluid whose flow rate is to be measured by the ultrasound flowmeter is so hot that its temperature is above the Curie temperature of the piezoelectric crystal, any reliable operation requires a certain thermal insulation of the ultrasound transducer from the hot fluid. It is for that reason that flowmeters are equipped with ultrasound transducers so configured as to leave a spatial clearance between the ultrasound transducer and the hot fluid. The ultrasound waveguide in that configuration must ensure the best possible thermal insulation between the ultrasound transducer and the hot fluid while at the same time optimizing a loss-free and unimpeded transfer of the ultrasound waves. In other words, the design of an ultrasound waveguide where the ultrasound transducer is located at a distance from the hot fluid or at least thermally insulated from that fluid enables the ultrasound transducer to inject ultrasound waves generated by it into the moving fluid and to receive the ultrasound waves from the hot fluid.
Conventional flowmeters equipped with an ultrasound transducer and an associated ultrasound waveguide employ ultrasound waveguides of the type as described, for instance, in WO 96/41157. The ultrasound waveguide according to that design incorporates multiple, mutually parallel, very thin rods, with the diameter of the individual rods being significantly smaller than the wavelength of the ultrasound signal to be conducted. The rods are typically bundled closely together and fitted into a tube that supports them laterally and constitutes a jacket for the ultrasound waveguide, thus making for a compact ultrasound waveguide design.
WO 96/41157 also describes an ultrasound waveguide design in which metal plates, bent in an essentially circular shape, are interleaved at a distance from one another. These rounded metal plates are again housed in a tube that constitutes the outer enclosure of the ultrasound waveguide.
EP 1 098 295 on its part describes an ultrasound waveguide that consists of a rolled-up foil fitted tightly into a metal tube. For transmitting ultrasound waves in the frequency range from 15 kHz to 20 kHz, the thickness of the foil layers is less than 0.1 mm. The foil typically consists of a metallic material.
According to the generally applied design concept, the ultrasound transducer is positioned at one end of the ultrasound waveguide in such fashion that the ultrasound transducer can feed ultrasound waves into, and receive them from, the ultrasound waveguide. The ultrasound transducer is usually plugged into one end of the ultrasound waveguide and thus is in direct physical contact with it. In the case of the aforementioned ultrasound waveguide with the rolled-up foil according to EP 1 098 295, the ends of the ultrasound waveguide are usually welded up and faced, and the ultrasound transducer is mounted on that welded, flat end face of the ultrasound waveguide.
As stated above, the ultrasound waveguide in flowmeters of the type to which this invention relates protrudes at least partly into the measuring tube. This allows the ultrasound waveguide to make direct contact with the fluid into which ultrasound waves are to be injected and from which ultrasound waves are to be received. That, however, poses a problem insofar as the measuring tube with the inserted ultrasound transducer must be sealed toward the outside. One way to solve that is to fit the jacket of the ultrasound waveguide directly into the measuring tube and seal it for instance by welding. Another way is to fit the ultrasound waveguide into a flange, again sealed for instance by welding, and to then attach that flange to a corresponding flange on the measuring tube, with the seal provided between the two flanges.
If the ultrasound waveguide is at least partly inserted into the measuring tube via a flange, a corresponding adapter may be provided on the measuring tube. In that case the waveguide does not have to extend into the measuring tube past the outer wall of the latter. Nevertheless, as intended by this invention, the waveguide protrudes at least partly into the measuring tube due to the fact that the waveguide is positioned in the cavity created by the adapter that connects directly to the interior of the measuring tube. In other words, the invention is not limited to configurations where the waveguide is at least partly inserted into the measuring tube in a manner whereby it protrudes into the interior of the measuring tube itself. Indeed, the waveguide may even be set back from the measuring tube proper. All that matters is that the waveguide, by virtue of its at least partial insertion in the measuring tube, is at least in indirect contact with the fluid passing through the measuring tube.
What poses a problem, however, is the fact that mounting a flange on the measuring tube, typically via an adapter, creates the cavity referred to above, which could interfere with the flow of the fluid in the measuring tube. In other words, the fluid might penetrate into the cavity created by the adapter and thus all the way to the seal between the two flanges which, in turn, requires a seal that can withstand the temperature of what may be a very hot fluid. This limits the available options for the gasket material while at the same time having a negative effect on the suppression of cross coupling, or a phenomenon referred to as crosstalk.
The problem with the flowmeter designs discussed is that the ultrasound waves generated by the ultrasound transducer, when transmitted, are injected not only into the ultrasound waveguide but also into the jacket encasing the ultrasound waveguide. Conversely, when the ultrasound transducer is intended to detect ultrasound waves, the ultrasound waves reach the ultrasound transducer not only via the ultrasound waveguide but via the jacket as well. It follows that the ultrasound waves transmitted or detected by the ultrasound transducer are waves transferred by both the ultrasound waveguide and the surrounding jacket. Now if it is by way of its jacket that the ultrasound waveguide is installed in and connected to the wall of the measuring tube carrying the fluid whose flow rate is to be determined, not only the ultrasound waves passing through the fluid but also those ultrasound waves that travel through the wall of the tube to and from the ultrasound transducer will be collected. The resulting crosstalk phenomenon may lead to a hetero-dyning or even total disruption of the measuring signal that is actually of interest.
The problem associated with that is highlighted by the realization that even when switching between two different fluids, and disregarding any geometry-related effects, the ultrasound transmission coefficient will be:
T=4(z1/z2)/(1+z1/z2)2
where z1 and z2 are the characteristic im fluid between which the ultrasound waves are switched. For a transition from steel to air, the aforementioned transmission coefficient T is approximately 0.004%, meaning that a significant part of the acoustic energy, 99.996% to be exact, is lost. A substantial component of this lost energy reappears in the form of undesirable crosstalk. It follows that this crosstalk is a key factor in the determination of the signal-to-noise ratio of the flowmeters discussed here.
It is therefore the objective of this invention to provide a flowmeter of the type described above, so improved in design that cross coupling and crosstalk are further reduced and at the same time any interference with the flow of the fluid through the measuring tube is minimized.
With reference to the flowmeter design first above described, this invention achieves that objective by positioning the seal between the ultrasound waveguide and the measuring tube. Thus, according to the invention, the ultrasound waveguide is not directly connected to the measuring tube and the seal is not located between a flange to which the ultrasound waveguide is attached and some corresponding flange on the measuring tube.
The approach according to this invention whereby the seal is positioned between the ultrasound waveguide and the measuring tube, offers several advantages. For one, the seal that consists of a material other than that of the measuring tube, reduces the transfer of acoustic energy as compared to a situation where the ultrasound waveguide is mounted directly into the wall of the measuring tube. In other words, the coefficient of transmission at this transition point is very small. Moreover, the concept according to this invention makes it possible even in cases where the measuring tube is provided with an adapter, to install the seal in such fashion that next to no additional cavity space is created in the measuring tube. Consequently, there is virtually no turbulence caused in the fluid passing through the measuring tube, which, in turn, enhances the measuring accuracy of the flowmeter.
A particularly tight seal is obtained in a preferred refinement of this invention with a seal that applies radial pressure on the ultrasound waveguide. Preferably, therefore, the sealing gasket is attached along the perimeter of the ultrasound waveguide.
The seal according to this invention can be established in a manner whereby the gasket is placed between the ultrasound waveguide and the measuring tube proper. However, in a preferred embodiment of this invention, the measuring tube is provided with an adapter, the ultrasound waveguide is inserted in the measuring tube via that adapter and the seal is placed between the ultrasound waveguide and the adapter. The adapter may be solidly connected to the measuring tube. In a preferred embodiment of this invention, however, the adapter is detachably mounted on the measuring tube, facilitating the assembly process.
This invention further relates to a flowmeter with a measuring tube, an ultrasound transducer, an ultrasound transducer mount and a seal, with the ultrasound transducer so attached to the ultrasound transducer mount, and the ultrasound transducer mount at least partly inserted in the measuring tube, such that ultrasound waves generated by the ultrasound transducer can be transmitted via the ultrasound transducer mount into a fluid passing through the measuring tube and/or, conversely, ultrasound waves emanating from the fluid can be transferred via the ultrasound transducer mount to the ultrasound transducer.
A flowmeter with an ultrasound transducer mount would pose problems similar to those associated with flowmeters, as described further above, employing an ultrasound waveguide.
Accordingly, the invention is also aimed at improving the flowmeter design described above so as to further reduce cross coupling and crosstalk while at the same time minimizing any interference with the flow of the fluid through the measuring tube.
Progressing from the above-described flowmeter design with an ultrasound transducer mount, this objective is achieved by positioning the seal between the ultrasound transducer mount and the measuring tube.
According to a preferred, enhanced implementation of the invention, the seal is again installed in such a way as to press radially against the ultrasound transducer mount and preferably against the perimeter of the ultrasound transducer mount.
For both the flowmeter with an ultrasound waveguide and the flowmeter with an ultrasound transducer mount, the seal in a preferred implementation of the invention is in the form of a ring-shaped gasket. Other gasket shapes are essentially possible but an annular gasket provides a particularly good seal. Particular preference is given to a seal in the form of a packing made up of multiple annular gaskets. Best suited is a packing of annular seals of the type used in other technical fields as well, such as a valve-stem packing for sealing a longitudinally moving valve stem. Of course, only the stationary sealing performance of the annular packing is important; the ability of a valve-stem packing to seal even a longitudinally moving stem is not critical in this case since the ultrasound waveguide and the ultrasound transducer mount discussed here remain essentially motionless during the operation of the flowmeter, i.e. they remain stationary. Only temperature fluctuations could cause minor shifts.
While in essence the annular seals could consist of any one of a great many suitable materials, the annular seals in a preferred embodiment per this invention consist of graphite, polytetrafluoroethylene (PTFE) and/or perfluoroelastomer. These materials have proved to be particularly reliable when used for very hot and/or chemically aggressive fluids.
When one or several annular gaskets are provided, there is generally no need for any further sealing provisions. However, in a further, preferred refinement of the invention, the packing with one or several annular gaskets is pressure-loaded in a vertical direction relative to the plane of the seal. This is preferably accomplished by means of a set of spring washers which are positioned in a way that, held in place by a stop on the far side of the seal, they exert top or, respectively, bottom pressure on the seal which on its part is held in place by a lug. This compresses the material of the seal in the direction in which the pressure is exerted and the material gives in a perpendicular direction, i.e. both toward the inside and the outside, further improving the sealing effect by virtue of the pressure bearing on the ultrasound waveguide or ultrasound transducer mount, as the case may be. According to another refinement of the invention, the sealing effect of the pressure-loaded seal is further improved by using annular gaskets with a V-shaped i.e. flared cross section. This flared profile subserves the mentioned inward and outward give of the sealing material, thus further enhancing the sealing effect. On the other hand, gaskets with a triangular cross section have also proved to be effective.